1. Field of the Invention
The present invention relates to an optical recording (storage) medium and particularly to an optical recording medium operated under a sampled servo scheme.
2. Description of Related Art
An optical disc of so-called MASS (Multiplexed Address Sampled Servo) format is known as an optical recording medium suited for a sampled servo scheme.
FIG. 1 of the accompanying drawings illustrates a format of a conventional optical disc of MASS format.
In this drawing, servo regions having various kinds of pits formed therein, which are used by a reproduction apparatus when performing servo control, and data regions for recording digital data are alternately provided on respective recording tracks. The servo and data regions are aligned in a radial direction of the disc. For the sake of simplification, radial cosine pits Prc and wobble pits Pwbl (will be described below) are only indicated in the servo regions in FIG. 1.
As illustrated in FIG. 1, a single radial cosine pit Prc is formed on each recording track in each servo region, and takes one of two different positions in the information scanning (reading) direction. Two wobble pits Pwbl are formed for each recording track in each servo region such that they are spaced from the associated recording track in the disc radial direction. Specifically, one wobble pit Pwbl is spaced upwards from the recording track by a predetermined distance and the other wobble pit Pwbl is spaced downwards from the same recording track at the same distance.
The pit pattern of the radial cosine pits Prc and wobble pits Pwbl does not change on the same track, but differs from the pit pattern on a next track spaced in the radial direction of the optical disc. That is, as shown in FIG. 1, the radial cosine pits Prc are positioned relatively to the right (Position 2) on Recording Tracks 1 and 3, the left wobble pits Pwbl of Recording Tracks 1 and 3 are spaced upwards from Recording Tracks 1 and 3 and the right wobble pits Pwbl are spaced downwards from Recording Tracks 1 and 3. On the other hand, the radial cosine pits Prc are positioned relatively to the left on Recording Tracks 2 and 4, the left wobble pits Pwbl of Recording Tracks 2 and 4 are spaced downwards from Recording tracks 2 and 4 and the right wobble pits Pwbl are spaced upwards from Recording tracks 2 and 4. The wobble pits Pwbl of neighboring recording tracks on the optical disc of MASS format overlap each other as illustrated in FIG. 1.
Referring to FIG. 2, illustrated is a detailed arrangement of the pits in the servo region.
As depicted in this drawing, the radial cosine pits Prc alternately take one of the two positions (Position 1 or 2), which are shifted from each other in the information scanning direction, on the succeeding recording tracks. By taking advantage of this position difference of the radial cosine pits, a reproduction apparatus for reproducing recorded information determines whether the recorded information is now being read from an odd number recording track or an even number recording track. Clock pits Pclk are aligned in the disc radial direction and serve as references for the reproduction apparatus when performing clock retrieval. A mirror portion between the radial cosine pit Prc and clock pit Pclk is used as a synchronization reference for the reproduction apparatus. The wobble pits Pwbl are spaced from the associated recording track upwards and downwards, and serve as references for the reproduction apparatus when performing the tracking servo control. A pair of address pits Padr are formed on each recording track in each servo region, and have a pit pattern that changes in a sixteen-recording-track cycle.
A scanning (reading) laser beam radiated from a pickup installed in the reproduction apparatus creates a beam spot BS on the recording surface of the optical disc. As the optical disc rotates, the beam spot BS traces the recording track indicated by the solid line in a direction indicated by the arrow in FIG. 2. The pickup receives a reflected light of the beam spot BS from the recording surface and obtains a scanned (read) signal in accordance to an amount of the reflected light. During this operation, the tracking servo is conducted such that the beam spot BS accurately traces the recording track. To this end, the beam spot BS is required to trace the recording track that extends on the center line between a pair of wobble pits Pwbl. The tracking servo is therefore performed in such a way that the scanned signals resulting from the two wobble pits Pwbl have the same level. It should be noted, however, that as shown in FIG. 2 the pit pattern of a pair of wobble pits Pwbl alternately changes with the recording track so that the reproduction apparatus utilizes the radial cosine pits Prc to determine which recording track is now being read, and alternately reverses polarity of a tracking error signal. By doing so, the beam spot BS can accurately trace the recording tracks.
If the pit length and pit interval are reduced to raise recording density of the optical disc, if a focus servo is inaccurate or if the disc becomes deformed and tilted, then adjacent pits on the same recording track influence each other and waveforms of read signals become distorted. This is called intersymbol interference. Variations of the waveforms caused by the intersymbol interference depend upon the pit pattern defined by adjacent pits on the same recording track. The reproduction apparatus detects from the waveform of the read signal a servo error signal in the servo loop such as the tracking servo loop or PLL (Phase Locked Loop) for clock retrieval. If the above mentioned intersymbol interference occurs, the level of the servo error signal also changes since the waveform of the read signal changes.
In the optical disc operated under the conventional sampled servo scheme, the pit pattern does not change in the respective servo regions along the same recording track. Therefore, even if the intersymbol interference occurs, the level of the servo error signal does not change for the same recording track, and disturbance of low frequency is added to the servo loop of the reproduction apparatus. In such a case, the servo loop (servo control) follows the low frequency disturbance so that the servo control becomes inaccurate.
The present invention intends to overcome the above described problems, and its primary object is to provide an optical recording medium that enables a reproduction apparatus to accurately perform tracking servo even if intersymbol interference occurs.
According to one aspect of the present invention, there is provided an optical recording medium comprising a plurality of frames continuously extending in an information reading direction, each frame being defined by a servo region in which tracking pits are formed for tracking servo control and a data region for recording information data, wherein a position of each of the respective tracking pits in each respective servo region is changed in the information reading direction between successive frames.